I was browsing Amazon the other day and saw a Lycamobile SIM card from several vendors...for 1 cent! Free shipping. I have no real need for one at the moment and will probably never use Lycamobile (a MVNO - Mobile Virtual Network Operator phone company that uses T-Mobile's network), but for a penny, why not snag one? There were a couple of cases in the last year where I wanted to test phones to see if the are unlocked.
That's where one of these cheap SIM cards could help you: if you are planning to go overseas and buy a SIM card. But is your phone unlocked? Buy one of these penny SIM cards and you can easily find out. Just pop it in. You won't get actual phone service (unless you pay for a Lycamobile plan), but you'll quickly find out if your phone is locked or not: turn off the phone, swap in the Lycamobile SIM for your old one, turn on the phone...and see what happens. If you are promoted for a code to unlock your phone, you know it is locked.
And if you have an unlock code (from your phone provider) you were going to try when you bought a SIM overseas? You can type it now! You need to type in this unlock code only once to unlock the phone forever. But why not do it ahead of time to make sure the code actually works, before you leave home? You can throw the SIM away after you've used it for the test.
To buy a Lycamobile SIM, search Amazon for "lycamobile sim card." You'll get a bunch of results. A few of them will be for one cent with free shipping (Prime not needed - I don't have it). There's no catch - you'll be charged only a penny. The SIM I got is a 3-in-1 SIM that will work as either a standard SIM, a micro SIM or nano SIM - should fit any phone.
Disclaimer: this test probably won't work if your phone is locked to the T-Mobile network, because Lycamobile uses T-Mobile. If you have say a MetroPCS phone (uses T-Mobile), Lycamobile's SIM would work in the phone without unlocking it - so this Lycamobile SIM won't help you. In that case, look for an AT&T SIM (Cricket, FreedomPop, or any AT&T MNVO), just not another T-Mobile operator.